The present invention relates to a lead storage battery and, more particularly to a thin and sealed typed lead storage battery wherein an electro-collecting board of lead or lead alloy is arranged within a battery casing and a cover thereof, and active material layers are formed closely adjacent to the electro-collecting boards, and the thus produced casing and cover with the plates for the anode and cathode are connected together with a separator therebetween.
In a standard lead storage battery, a lead or lead alloy supporter; which is formed by a punching, a casting or an expanding method, is coated with active materials which are mixed with lead powder or lead monoxide powder and an aqueous solution of sulfuric acid and other additives, and then subjected to curing process in a vapor atmosphere, and to a drying process to form positive and negative electrodes. The thus produced electrodes are combined with a porous separator and then inserted into the casing made of synthetic resins, and followed by a sealing step of the casing to thereby complete the lead storage battery.
The conventional lead storage battery as described has been extensively used for its reliability in quality and economic aspects. Along with the recent miniaturization of devices and instruments in various fields, the miniaturization of batteries, such as lead storage batteries, for power sources of such devices and instruments is of interest and is sought after enthusiastically. In order to comply with the requirement of the industry, an attempt has been made to use a synthetic net as a supporter in place of the conventional metal in the form of a lattice and a lead plate or lead alloy plate was used for an electrocollecting board so that pastelike active materials are disposed to the synthetic resin net to form electrodes. The thus formed electrodes are cut to a predetermined size and then closely contacted to the collector to thereby complete a thin and light-weighed lead storage battery.
However, the lead storage battery in accordance with the above-mentioned attempt reveals some difficulties, which will be described hereinafter. The aforesaid electrodes attempt in producing is basically common to that of the conventional method, and more specifically, it is necessary that paste-like active materials be prepared and then provided onto the supporter for the synthetic resin net. Therefore, during the process of forming the active material, a noxious powder such as a lead powder is apt to be scattered in the air, resulting in an extreme deterioration of the active material. Further, it is difficult to make the density of the paste-like active material constant resulting in a scattering in the capacity of the electrodes. Particularly, since the quantity of the applied paste-like active material is limited in the thin-type battery, non-uniformity of the battery characteristics becomes much more serious in accordance with the uniformity of the electrodes in respect to the capacity thereof. Besides, in the production of the lead storage battery, troublesome assemblies and steps are necessary such as the preparation of paste-like active materials, coating of the active materials on the supporter, cutting it to a predetermined size, and assembling it to make it closely contacted with the electr-collecting board.
In addition, such a thin-type lead storage battery, which is to be mounted within a device or an instrument, must be sealed, as described in Japanese Patent Publication No. 50-12092 or Japanese Unexamined Patent publication No. 57-176667. The basic structure thereof will be explained with reference to FIG. 1 of the drawings. In FIG. 1, reference numerals 1 and 2 represent a battery casing of a synthetic resin and a battery cover 2 of a synthetic resin, respectively. Within the space 3 confined by the casing 1 and the cover 2, a negative electrode 4 and a positive electrode 5 are positioned through a separator 6. In such a thin-type storage battery, the conventional method wherein an electrolyte is supplied through a small hole of the casing after the cover and the casing are adhered together is not applicable for miniaturizing the batteries and producing them in quantity. Thus, a method has been used in which the casing and cover are subject to ultrasonic welding after the electrolyte is secured by the electrodes 4, 5 and the separator 6. The battery has an outlet valve 7 on the inner wall of the casing, an outlet port 8, and a valve seat 9, as illustrated.
However, the method by ultrasonic welding have revealed other difficulties. Namely, in the welding step of the casing and cover, the active materials positioned adjacent to the positive and negative electrodes 4, 5 are scattered due to vibration, and the scattered active material around the electrodes induces a short circuit of the positive and negative electrode. Similarly, the scattered active material is exhausted together with the electrode out of the casing 1 through the outlet port 8, and at this moment the scattered material is stuck to the position between the outlet valve 7 and the valve seat 9, resulting in a failure of the sealing at this part of the battery. In order to avoid such a short circuit, a separator having a larger areas than the electrodes is used to provide an electrical separation between the electrodes. Further, a sufficient space 10 is formed between the electrodes and the outlet port 8 so that a longer passage is formed between these elements in order to minimize the quantity of the active material which is exhausted through the outlet port 8 and to minimize the sticking of the active material to the valve seat 9. However, the provision of the relative large space 10 and larger separator reduces a volume efficiency of the battery and results in a failure of miniaturization.
A mechanism for permitting a gas, which is generated by an electrochemical reaction within the lead storage battery, to be exhausted by means of a safety valve or gas exhaustion hole is known in, for example, Japanese Utility Model Publication Nos. 40-31878, and 40-31879 and Japanese Unexamined Utility Model Publication No. 58-155763. In Japanese Utility Model Pub. No. 40-31878, as illustrated in FIGS. 2A-2C, the casing 11 has at its upper portion a cylindrical projection 12, an aperture 13 communicated with the interior of the casing, and a rubber tube 14 fitted to the cylindrical projection to close or plug the aperture 13. A cover (not shown) prevents the rubber tube 14 from being dislocated from the cylindrical projection, wherein the cover has an aperture (not shown). In Japanese Utility Model Pub. No. 40-31878, as illustrated in FIGS. 3A-3C, a cylindrical projection 16 with an opening 17 is formed on top of the casing 15 and a rubber cap 18 is adapted to the projection to close the opening 17. The cap is held by a cover (not shown) so that it is not removed or dislocated, wherein the cover has an aperture (not shown). In Japanese Unexamined Utility Model Publication No. 58-155763 a casing is formed with a pair of synthetic resin members and a frame interposed therebetween, and a safety valve port is provided for communication between the interior and outside of the casing.
However, the above-mentioned structure reveals the serious problems as described hereinafter. In the battery structure of Japanese Utility Model Pub. No. 40-31878, if it is applied to a thin-type lead storage battery, the cylindrical projection 10 must be formed on the side portion of the casing 9 so as to minimize the thickness of the casing, as shown in FIG. 4. Such a casing 9 with a cylindrical projection 9 is unitarily formed by molds which are moved in the direction as shown by arrows A and B (FIG. 4). The molds must have slidable rod for providing the cylindrical projection 10, and it is difficult to prepare such molds. By contrast the battery disclosed in Japanese Utility Model pub. No. 40-31879 permits a relatively easy production of the mold having a slidable rod, but is almost unapplicable to a battery which is not only small but also complex in shape since there is an inherent limitation of produceable shape and size due to working prevision of the molds, packing characteristics of resins and other factors such as shrinkage of the hardening resin. Japanese Unexamined Utility Model Pub. No. 58-155763 shows a structure in which circumferential portion of the safety valve is forcibly contacted with the frame and a pair of the resin members, and thus additional space for the valve chamber, resulting in an increase of the size in the longitudinal direction. Additionally, the safety valve must provisionally be fixed to the predetermined position within the valve chamber prior to setting of the resin members, and at this moment, the safety valve, which was provisionally press-fitted to the chamber, is displaced out of the chamber by its resilient recovery force. Thus, this structure presents difficulties in assembly operation.
In addition to the above, in the sealed type lead storage battery, a quantity of the electrolyte is generally restricted, and the concentration of the electrolyte is set to about 40 percent which is higher than that of a general storage battery for automobiles, wherein the electrolyte concentration at the time of charge is about 35 percent. The reason for the higher concentration of the electrolyte are as set forth below.
(1) Repetition of discharge to an extent that the active material is used up will shorten a life time for the battery and thus quantity of sulfuric acid in the electrolyte is restricted to restrictively determine the electrode reaction, with unused active material being maintained as it is, whereas such a problem as above is not raised with reference to the automobile battery in which charge and discharge proceed at the same time.
(2) For the purpose of obtaining higher energy density of the battery, unnecessary water is excluded as much as possible to provide higher volume efficiency and weight efficiency. Particularly, this requirement is remarkable for the thin type or small sized lead storage battery.
(3) In a sealed structure, oxygen gas generated from the positive electrode during charge is chemically consumed by the negative electrode. In such a chemical reaction, three phases (Pb for solid phase; electrolyte for liquid phase; and O.sub.2 for gaseous phase) are necessary, and this chemical reaction is delayed if the quantity of the electrolyte is greater than the normal. Therefore, the electrolyte is restricted in its quantity.
By the reasons described above, in the sealed type lead storage battery, the electrolyte thereof is set to have the highest concentration which is close to the upper limit of the allowed concentration for the lead storage batteries.
In the conventional sealed-type lead storage battery, the casing and the cover are made of ABS resins from the viewpoints of mechanical strength, dimensional stability, surface appearance, ecconomical aspects, etc. However, such resins as ABS resins has a vapored water permeable property, and water in the electrolyte is vapored to penetrate the casing wall or cover wall to lead out of the battery. Consequently, the concentration of the electrolyte becomes higher than the aforementioned limitation, and such a high concentration of the electrolyte accelerates the corrosion speed of the electrodes plates resulting in the deterioration of the electrode plate capacity and the life of the battery. Up to the present, no specific countermeasure has been successfully made to alleviate the exceedingly high concentration of the electrolyte due to the abovementioned phenomena, presumably because the property level of the conventional batteries is rather low. Although an attempt was made to use a resin having a relatively low vapored water permeability such as a high-impact polystyrene, the above-mentioned problems have not yet been solved successfully.